JPH06179945A - Cr-mo series ultrahigh tensile strength electric resistance welded steel tube excellent in ductility - Google Patents

Abstract

PURPOSE:To provide a Cr-Mo series ultrahigh tensile strength electric resistance welded steel tube excellent in ductility and used for an automotive structural member or the like. CONSTITUTION:This is an electric resistance welded steel tube having a componental compsn. obtd. by incorporating, if required, one or more kinds of, by weight, 0.01 to 0.2O% Ti and 0.01 to 0.20% Nb. into 0.15 to 0.30% C, 0.05 to 0.50% Si, 2.0 to 3.0% Mn, 0.005 to 0.O20% P, 0.0005 to 0.0060% S, 0.01 to 0.08% Al, 0.001 to 0.0O3% B, 0.002 to 0.0050% N, 0.1 to 0.7% Cr and 0.1 to 1.5% Mo, and the balance Fe with inevitable elements, and in which normalizing is executed as heat treatment to regulate its tensile strength into 150 to 180kgf/mm<2>, elongation into >=10% and yield ratio into 0.70 to 0.85.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high-strength electric resistance welded steel pipe used for structural members of automobiles, etc., and particularly economical because it has a tensile strength of 150 kgf / mm ² or more for door reinforcement and an elongation of 10% or more. The present invention relates to a Cr-Mo-based ultra high-strength electric resistance welded steel pipe having excellent ductility.

[0002]

2. Description of the Related Art Regarding structural members such as automobiles, thorough weight reduction is being considered for improving fuel efficiency and environmental measures. As one measure to achieve both safety and safety, some members have a weight of 150 kgf / mm. More than ² ultra high strength steel pipes are being adopted.

As a steel pipe material for reinforcing automobile doors, a method of tempering after electric resistance welding, that is, quenching or quenching and tempering, as described in JP-A-3-122219, etc. A method of normalizing a predetermined low alloy steel such as 3-140441 is common.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
There are two types as described in the prior art section.
First, in the method as described in JP-A-3-122219, special heat treatment equipment for processing each piece is required, and special attention is required to secure the dimensions and shape, and the productivity. Is low and the cost is extremely high in terms of capital investment and productivity. Further, it is unsuitable as a method for manufacturing a square steel pipe or a deformed steel pipe, which is attracting attention as a structural member having higher rigidity, in terms of ensuring dimensional accuracy.

Next, a method of normalizing a predetermined low alloy steel such as Japanese Patent Laid-Open No. 3-140441 can solve the above-mentioned problems of the quenching type, but depending on the component, the material cost becomes high and the ductility deteriorates. There is a case. JP-A-3-14044
In the case of 1, Mn exceeds 3%, production in a converter is practically impossible, and expensive components such as Ni are contained. Moreover, the manufacturing method and elongation of the steel pipe are not disclosed. The present invention is a normalizing type, more economical than conventional methods, easy to manufacture, and has a tensile strength of 150 kgf.
/ Mm ² or more and excellent Cr-M with a ductility of 10% or more
It is intended to provide an o-type ultra-high-strength electric resistance welded steel pipe.

[0006]

The subject matter of the present invention is as follows.

(1) Component composition by weight C: 0.15 to 0.15
0.40%, Si: 0.05 to 0.50%, Mn: 2.
0-3.0%, P: 0.005-0.020%, S:
0.0005-0.006%, Al: 0.01-0.0
8%, B: 0.0010 to 0.0030%, N: 0.0
02-0.005%, Cr: 0.1-0.7%, Mo:
An electric resistance welded steel pipe containing 0.1 to 1.5% of balance Fe and unavoidable elements, consisting of a structure mainly composed of martensite and bainite by normalizing, and a tensile strength of 150 to 180 k.
Elongation is 10% or more at gf / mm ² , yield ratio is 0.70
Cr-M with excellent ductility characterized by 0.85
o Super high tensile ERW steel pipe.

(2) The component composition by weight is C: 0.15 to 0.15.
0.40%, Si: 0.05 to 0.50%, Mn: 2.
0-3.0%, P: 0.005-0.020%, S:
0.0005-0.006%, Al: 0.01-0.0
8%, B: 0.0010 to 0.0030%, N: 0.0
02-0.005%, Cr: 0.1-0.7%, Mo:
0.1 to 1.5%, Ti: 0.01 to 0.20%, N
b: An electric resistance welded steel tube containing 0.01 to 0.20% or less of one or two kinds and the balance Fe and unavoidable elements, which is composed of a structure mainly composed of martensite and bainite by normalization and has a tensile strength of 150. A Cr-Mo super-high-strength electric resistance welded steel pipe having excellent ductility, characterized in that the elongation is 10% or more at -180 kgf / mm ² , and the yield ratio is 0.70 to 0.85.

(3) Cr having excellent ductility according to the above (1) or (2), wherein the electric resistance welded steel pipe is a square or deformed steel pipe.
-Mo-based ultra-high-strength electric resistance welded steel pipe.

The present invention will be described in detail below. First, the reasons for limiting the components common to claims 1 to 3 among the components of the steel sheet used in the present invention will be described.

If the amount of C is small, the ductility is good and the workability is excellent, but the required strength cannot be obtained, so the lower limit was made 0.15%. On the other hand, if it exceeds 0.40%, cold workability such as formability at the time of pipe making and toughness tend to be deteriorated, and the heat-affected zone is hardened during pipe making welding of the electric resistance welded steel pipe, resulting in cutting or the like. The upper limit was set to 0.40% because it causes problems.

In the case of killed steel, it is difficult to keep Si less than 0.05% in terms of steelmaking technology, and if it exceeds 0.5%, the electric resistance weldability and toughness deteriorate, so 0.5% is the upper limit. And

[0013] Mn has a good balance between strength and ductility, and is required to be at least 2.0% or more in order to increase strength and secure elongation. If it exceeds 3.0%, melting in a converter becomes extremely difficult, so the lower limit is 2.0% and the upper limit is 3.0%.
And

P is an element that is inevitably mixed during steelmaking, but it is difficult to make P less than 0.005% because of steelmaking technology.
If it exceeds 0.020%, weld cracking tends to occur particularly during electric resistance welding of ultra-high strength steel pipe, so the lower limit is 0.005%,
The upper limit was 0.020%.

Like P, S is an element that is unavoidably mixed during steelmaking, and it is difficult to make S less than 0.0005% in steelmaking technology. If it exceeds 0.0060%, weld cracking tends to occur during electric resistance welding. Therefore, the lower limit was made 0.0005% and the upper limit was made 0.0060%. In order to further suppress cracking during electric resistance welding with S, Ca, which is an element that controls the morphology of MnS, may be added.

In the case of killed steel, it is difficult to control Al to less than 0.01% in terms of steelmaking technology, and when it exceeds 0.08%, cracks in cast slabs, internal defects due to formation of large oxide inclusions, etc. Therefore, the lower limit is 0.01% and the upper limit is 0.08%.

B is an essential element for delaying the ferrite transformation in the cooling process to obtain a high-strength transformation structure, but also in the composition of the steel of the present invention, if it is less than 0.001%, the strength becomes insufficient, and 0.003%. Boron beyond
Constitutant forms and ductility decreases significantly, so the lower limit is 0.001% and the upper limit is 0.003%.
And

N is an element that is unavoidably mixed during steelmaking, but it is difficult to keep it less than 0.002% in steelmaking technology, and if it exceeds 0.005%, the strength increasing effect of Ti and B is impaired and the strength is increased. The lower limit is 0.002 to cause a shortage.
%, And the upper limit was 0.005%.

Mo suppresses ferrite transformation, has an effect of grain refining, and has a characteristic of precipitation strengthening. By heat treatment after pipe forming, a structure mainly composed of martensite and bainite is obtained,
Since it is effective for increasing the strength, 0.1% or more is contained. However, even if added in excess of 1.5%, the improvement of the effect is small and the ductility is deteriorated.
%, And the upper limit was 1.5%.

Cr is a relatively economical component and is effective in suppressing ferrite transformation and obtaining a bainite structure containing martensite by heat treatment after pipe forming and increasing the strength. The above is included. in this case,
If added in excess of 0.7%, welding defects due to Cr oxides are likely to occur in ERW pipe making, and cumbersome inert gas seal welding is required. Therefore, the upper limit is set to 0.7%.

Similar to Mo, Ti and Nb have the effect of refining because they expand the unrecrystallized γ region in hot rolling, and strengthen precipitation, both of which are elements that increase the strength of the steel material. , Because it is effective in the production of ultra-high-strength ERW steel pipe,
You may contain 0.01% or more. But 0.20
%, The ductility is impaired, so the lower limits of Ti and Nb are 0.01% and the upper limits are 0.20%.

Next, the manufacturing process will be described. An example of the manufacturing process of the electric resistance welded steel pipe according to the present invention is shown in FIG. In accordance with the present invention, the steel of the above components is 950 ° C. or less during hot plate rolling.
It is desirable to finish the finish rolling above the r ₃ transformation point.
This is necessary especially when an improvement in toughness is desired and when a low strength steel plate is obtained to facilitate pipe making. 95
Above 0 ° C, there is no rolling in the unrecrystallized region, so strength
The ductility is deteriorated, and if it is less than the Ar ₃ transformation point, the strength is increased by the two-phase rolling, but the ductility is remarkably reduced. Therefore, by finishing rolling the steel having the above components at a temperature of 950 ° C. or lower and at an Ar ₃ transformation point or higher during hot strip thickness rolling and continuously winding the steel under the conditions of the present invention, low strength and ductility that is easy to manufacture in a post process is obtained. It can be an excellent material.

When the coiling temperature is 600 ° C. or higher, the cooling rate in the coil is close to that of the furnace cooling, so that the precipitation of Mo and the like is overaged and the ferrite precipitates, which has relatively low strength and ductility. Can manufacture steel sheets. The steel sheet manufactured in this manner has sufficient ductility for forming an electric resistance welded pipe.

After pipe making, normalization is performed as a heat treatment. This is heated to the Ac ₃ point or more and austenitized, and then cooled in the same manner as air-cooling to suppress the formation of ferrite, thereby forming a structure mainly composed of martensite and bainite to increase the strength.
The normalizing temperature is preferably Ac ₃ + 20 ° C. or higher in consideration of temperature variation, and the upper limit is preferably Ac ₃ + 70 ° C. or lower in order to maintain fine grain and balance strength ductility. The air cooling here has a cooling rate up to 300 ° C. in the range of 10 to 150 ° C./min. If the heat treatment is performed at an Ac < ₃ point, the above effect cannot be obtained, and the desired strength cannot be obtained.

Although the electric resistance welded steel pipes according to claims 1 and 2 of the present invention have been described above, the electric resistance welded steel pipe according to claim 3 may be used. FIG. 2 shows steps according to the method of claim 3. By adding cold drawing as described above, it is possible to manufacture square steel pipes and deformed steel pipes having excellent bending strength. FIG. 3 shows examples of the shapes of the rectangular and deformed steel pipes. Cold drawing includes a method using die drawing and a method using roll forming. The base pipe heat treatment is to remove the processing strain caused by cold working during pipe making, soften the quench-hardened portion of the electric resistance welded portion, and improve the cold drawing workability.
Softening annealing or normalization at 00 ° C. or higher is performed. After cold drawing, normalizing is performed to remove cold work strain and improve the balance of strength and ductility. However, when normalizing is performed as the heat treatment of the tube, the strength has already risen sufficiently,
The finish heat treatment after cold drawing may be annealing. In this way, an appropriate strength-ductility balance can be obtained by combining the work hardening amount by cold working and the annealing temperature. Although the annealing temperature depends on the cold working rate, it is effective from 450 ° C or higher.

[0026]

EXAMPLES Table 1 shows conditions and results for producing electric resistance welded steel pipes having a size of φ34.1 × t2.0 mm by the conventional method and the method of the present invention. The cold drawn tube here was drawn into a square shape using a die. 150 kgf / m in the conventional method
Even if the tensile strength of m ² or more is achieved, the elongation cannot reach 10%, but it can be achieved by the method of the present invention. Further, according to the present invention, the yield ratio (= yield strength / tensile strength) is 0.70 to
It can be as low as 0.85. Buckling during bending is less likely to occur as the yield ratio is lower, so the bending absorbed energy is large, and the present invention is advantageous. Further, according to the present invention, a super-high-strength electric resistance welded steel pipe having a uniform base material / welded portion and an excellent balance of strength and ductility can be obtained by applying normalizing heat treatment after pipe making. Since various dimensions can be easily manufactured by adding cold pipe drawing after heat treatment, small lots can be handled and it is economical. Further, since it can be processed into an arbitrary cross section, a square tube or the like having excellent bending workability can be manufactured. Further, by performing normalization after cold drawing, the ductility is increased and the strength-ductility balance is improved. Further, if necessary, by appropriately controlling the finish rolling temperature and the coiling temperature in the hot plate thickness rolling, it is possible to manufacture a raw material steel plate having low strength and excellent ductility and facilitate pipe making. . Note that, although cold rolling was performed in this example, the point is that if cold drawing is performed, an increase in strength can be obtained by work hardening.
The same effect can be obtained by roll forming.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

EFFECTS OF THE INVENTION According to the present invention, there is no need for quenching, quenching, or tempering after an electric resistance welded pipe, and no special heat treatment equipment for treating each piece is required, which is economical. Furthermore, square steel pipes and deformed steel pipes, which are attracting attention as structural members having higher rigidity, can be easily manufactured. Also, compared to the conventional normalizing type, it can be manufactured in a converter,
It is economical because it does not contain expensive alloys such as Ni. Furthermore, the tensile strength is 150 kgf /
It has an excellent strength-elongation balance of mm ² or more and an elongation of 10% or more, and it becomes possible to manufacture an ultra-high-strength electric resistance welded steel pipe having a low yield ratio of 0.70 to 0.85, which is industrially possible. The contribution is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a manufacturing process of an electric resistance welded steel pipe according to claim 1 and claim 2 of the present invention.

FIG. 2 is a diagram showing an example of a manufacturing process of the electric resistance welded steel pipe according to claim 3;

FIG. 3 is a view showing an example of the shapes of a rectangular steel pipe and a deformed steel pipe of the present invention.

Claims (3)

[Claims] 1. Component composition by weight C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 2.0 to 3.0%, P: 0.005 to 0 0.020%, S: 0.0005 to 0.006%, Al: 0.01 to 0.08%, B: 0.0010 to 0.0030%, N: 0.002 to 0.005%, Cr: An electric resistance welded steel pipe containing 0.1 to 0.7%, Mo: 0.1 to 1.5% and the balance Fe and inevitable elements, which has a structure mainly composed of martensite and bainite by normalizing and has a tensile strength. Is 150 to 180 kgf / mm ² , elongation is 10% or more, and yield ratio is 0.70 to 0.85. Cr-Mo super-high strength electric resistance welded steel pipe having excellent ductility. 2. The component composition by weight is C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 2.0 to 3.0%, P: 0.005 to 0. 0.020%, S: 0.0005 to 0.006%, Al: 0.01 to 0.08%, B: 0.0010 to 0.0030%, N: 0.002 to 0.005%, Cr: 0.1 to 0.7%, Mo: 0.1 to 1.5%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.2
An electric resistance welded steel tube containing 1% or 2% of 0% or less and the balance Fe and unavoidable elements, which is composed of a structure mainly composed of martensite and bainite by normalizing and has a tensile strength of 150 to
Cr-Mo super-high-strength electric resistance welded steel pipe excellent in ductility, which is characterized by having an elongation of 180% or more at 180 kgf / mm ² and a yield ratio of 0.70 to 0.85. 3. The C having excellent ductility according to claim 1 or 2, wherein the electric resistance welded steel pipe is a square or deformed steel pipe.
r-Mo super-high strength ERW steel pipe.